Most detrimental substances, which contain e.g. heavy metals, are deposited on landfills in an unprocessed condition, which is hazardous for the environment. Hence the demand for efficient methods of processing and immobilization of such materials is very strong.
The immobilization of detrimental substances is effected primarily by a chemical transformation of these substances, which will stop them from being washed out in the form of dissolved solids, and if possible, by changing some of their physical parameters, so as to improve the mechanical durability of the product and decrease its water permeability.
The most popular method of stabilization of detrimental substances consists in mixing them with cement. Solidification using only cement produces merely the “outer barrier” resulting from morphology of crystals which create a physical structure. Migration is stopped as a result of fixation of waste in the bulk of cement, reduced permeability and increased density of the mixture. The drawbacks for solidification using only the cement are: the huge growth of volume (and mass) of the mixture as a result of large amounts of cement added, limited durability of the solid phase, and large porosity. Crystal build-up while the cement is added results from reaction of four significant components of cement. Hydration is caused by: tricalcium silicate (20-60%), dicalcium silicate (20-30%), tricalcium aluminate (5-10%) and tetra-calcium aluminoferrite (8-15%). Adding water leads to emergence of calcium hydroxide Ca(OH)2 and, consequently, the crystals. As the reaction takes place quickly, the resulting material is porous and therefore water-permeable. When water permeates through pores in the presence of acid rains and CO2, the change of pH caused by free Ca(OH)2 is restrained only in the initial phase. After some time the leaching of heavy metals resumes and permissible levels of these metals in water extract are exceeded. The period of full immobilization for cement is assumed at 2 to 3 years, depending on the quantity and quality of the cement used and atmospheric conditions.
In view of the above, many attempts are being made to discover a cost-efficient and productive way to permanently stabilize the hazardous substances.
Another preferable additive is sulphuric acid or magnesium sulphate or aluminium sulphate. Additives such as organic acids or sulphuric acid ensure that what is actually fixated, is a particular compound of the waste material, rather than the entire mixture. However in consequence, the resulting product fails to meet the limit values of parameters specified by environmental legislation, which is required to assume that the product has been neutralized effectively. This applies, in particular, to contents of sulphates or solid-state soluble substances. As a result the final product may be deposited only on a waste landfill. Additionally, this method fails to ensure the control over the reactions taking place during binding, which is a prerequisite for the efficient fixation of hazardous substances and compounds.
None of the methods of fixation of detrimental waste in use today ensure durability and stability of the resultant product, allowing it to be safely used.